A small nuclease resisting (nr) mouse chromatin fraction will be defined operationally as nuclease-resistant or resisting on the basis of experiments designed to compare the rate of degradation of total chromatin to the fraction of remaining nr chromatin. Characteristics of the nr chromatin DNA:sequence complexity, sequence organization and cytological location will be determined by DNA excess hybridization experiments as a function of single strand size and by in situ hybridization. Evolutionary conservation of the nr DNA for which there is preliminary evidence, will be investigated by heterologous DNA excess hybridizations with DNAs from a wide range of the phylogenetic spectrum, delta determinations and in situ hybridization to heterologous chromosomes. Nucleoprotein organization of nr chromatin will be investigated by: determining the histone and non-histone protein composition, nuclease digestion following treatment of nr chromatin with various reagents and psoralen cross-link distributions in the electron microscope. It will be determined whether this unusual chromatin fraction has a nucleosome organization.